Introduction
Okay, energetics. Big word, simple idea: does the reaction make stuff hotter or colder? Light a match, your fingers warm up. Crack a cold pack, it goes icy. Same deal, opposite directions.
Here's the cheat code for the whole Core chapter: a reaction either gives thermal energy out to its surroundings (they warm up) or takes thermal energy in from them (they cool down), and the thermometer snitches on which. Lock that in and every exam move here — name it, read the diagram, work out a temperature change, calculate the energy, design a fair test — is just that one idea in a different outfit. This is your fast refresh, not the full textbook: one key point per idea, one quick worked example, the exact move the marker wants. Let's go. 🔥🧊
Exothermic vs EndothermicC5.1
Every reaction either gives energy out or takes energy in. Exothermic = energy goes out to the surroundings, so they warm up (burning, neutralisation, metal + acid, respiration). Endothermic = energy gets pulled in from the surroundings, so they cool down (heating a carbonate, dissolving ammonium nitrate in a cold pack). You can't see energy, but you can read the thermometer: temp up = exothermic, temp down = endothermic. The surroundings basically tell on which way the energy went.
> 🔑 Key idea: Surroundings get hotter → energy out → exothermic. Surroundings get colder → energy in → endothermic. 🌡️
Worked Example: Heat Goes OutC5.1
Problem Acid is added to sodium hydroxide and the temperature rises from 21 °C to 32 °C. Exothermic or endothermic, and how do you know?
Step 1 Temperature change = final − initial = $32 - 21 = +11$, a rise of 11 °C.
Step 2 A rise means energy was released into the mixture → exothermic.
Chain it for the mark: temp rose → energy out → exothermic. ✅
Cold Means Energy InC5.1
When something dissolves and the beaker goes cold, that's the classic endothermic move. The dissolving is pulling energy out of the water, so the water cools. Cold to the touch doesn't mean "cold was made" (nothing makes cold) — it means the process soaked up energy from the surroundings. That's literally how an instant cold pack works: ammonium nitrate dissolving in water takes energy in and the bag turns icy.
> 🔑 Key idea: A mixture going cold = energy absorbed = endothermic. Cold ≠ cold released; cold = energy taken in. 🧊
Worked Example: The Cold BeakerC5.1
Problem A solid dissolves in water and the temperature falls from 20 °C to 13 °C. What's happening in terms of energy, and is it exothermic or endothermic?
Step 1 Temperature change = $13 - 20 = -7$, a fall of 7 °C → the water is losing energy.
Step 2 That energy goes into the dissolving, so energy is taken in → endothermic.
Energy in, temp down, endothermic. Done. ✅
Reaction Pathway DiagramsC5.1
A reaction pathway diagram puts the energy on one picture: energy up the side, "progress of reaction" along the bottom. For Core, one comparison does it all. **Products below reactants = exothermic (energy went out). Products above reactants = endothermic** (energy went in). The gap between the two levels is how much energy moved — bigger gap, more energy.
> 🔑 Key idea: Products below reactants = exothermic; above = endothermic. Bigger gap = more energy. 📈
Worked Example: Products Up HighC5.1
Problem On a pathway diagram the products are higher than the reactants. Exothermic or endothermic?
Step 1 Products higher → the chemicals gained energy, taken in from the surroundings.
Step 2 Energy in = endothermic.
Compare products to reactants, not to the bottom of the page. 🔺
Reading the Energy GapC5.1
The vertical gap on the diagram isn't just decoration — it's the amount of energy transferred. For an exothermic reaction (products lower), that gap is the energy released to the surroundings. The bigger the drop, the more energy comes out, and the bigger the temperature rise you'd measure. Two exo diagrams side by side? The one with the steeper drop wins on energy.
> 🔑 Key idea: The gap between reactant and product levels = energy transferred. Bigger drop = more energy released. ↕️
Worked Example: What's the Gap?C5.1
Problem Products are lower than the reactants. A gap labelled Q sits between the two levels. What is Q, and is the reaction exothermic or endothermic?
Step 1 Products below reactants → energy released → exothermic.
Step 2 The gap Q is the energy released to the surroundings.
Q = energy out, reaction = exothermic. 📏
Temperature ChangeC5.1
Nearly every number question starts here, and it's worth marks on its own. Temperature change = final temperature − initial temperature. Positive = it got hotter (exothermic); negative = it got colder (endothermic). When they only want the size, just take the difference and ignore the sign. Keep the same decimal places as the question, and subtract — never add (adding the two readings is the trap option, every time).
> 🔑 Key idea: Temperature change = final − initial. Subtract, don't add. Up = exo, down = endo. ➖
Worked Example: Just SubtractC5.1
Problem An acid's temperature rises from 24.0 °C to 46.5 °C. Work out the temperature change.
Step 1 change = final − initial = $46.5 - 24.0$.
Step 2 $46.5 - 24.0 = 22.5$, so the change is 22.5 °C.
Not 70.5 (that's adding), not 46.5 (that's the final reading). Subtract. ✅
Energy Transferred to WaterC5.1
Got a temperature change? You can find the energy given to the water. The formula: energy (J) = volume of water (cm³) × 4.2 × temperature change (°C). The 4.2 is just water's fixed number, and the answer is in joules. Put in the three numbers and multiply. The traps: use the volume of water (not the mass of any solid that dissolved) and the temperature change (not the final temperature), and don't divide by anything extra.
> 🔑 Key formula: energy (J) = volume of water × 4.2 × temperature change. Volume of water and the change — that's it. 🧮
Worked Example: Run the NumbersC5.1
Problem A reaction warms 15 cm³ of water by 13.5 °C. Work out the energy given out.
Step 1 energy = $13.5 \times 4.2 \times 15$.
Step 2 $13.5 \times 4.2 = 56.7$, then $56.7 \times 15 = 850.5$, so 850.5 J.
Use all three numbers — leaving out the volume is the classic slip. 🔢
Fair TestsC5.1
Loads of marks come from "describe how you'd compare…" two fuels or two reactions. The chemistry is easy; the marks are for making it fair — change only the thing you're testing, keep everything else the same. For fuels heating water: same volume of water, same start temperature, same distance to the flame, same container. Then change only the fuel and measure one thing — time to reach a set temp, or temperature rise in a fixed time.
> 🔑 Key idea: Fair test = keep everything the same except the one thing tested, and measure one outcome. ⚖️
Worked Example: Best FuelC5.1
Problem How do you find which of several alcohols heats water fastest, fairly?
Step 1 Keep it fair: same volume of water, same start temperature, same distance from the burner, same container.
Step 2 Change only the alcohol and time how long each takes to reach a chosen temperature.
The shortest time heats the water fastest. "Biggest flame" isn't a measurement. ⏱️
Reducing Heat LossC5.1
Real experiments lose heat to the air, which makes an exothermic temperature rise come out smaller than the true value (the reaction's still exothermic — your reading's just low). Fix it by losing less heat: wrap lagging (insulation) round the container, pop a lid on, or use a polystyrene cup instead of thin glass. Heat loss only ever makes the change look smaller — it never flips exothermic into endothermic.
> 🔑 Key idea: Heat loss makes the measured change too small. Lagging, a lid, or a polystyrene cup bring it closer to the true value. 🧥
Worked Example: Wrap It UpC5.1
Problem Heat is lost from a boiling tube in an exothermic reaction. State one change to make the measured temperature increase closer to the true value, and why.
Step 1 Heat lost → less energy warms the mixture → the measured increase is too low.
Step 2 Wrap lagging (insulation) round the tube: less heat escapes, so more stays in and the reading is closer to the true value.
A thinner tube would do the opposite — you want less heat loss. 🛡️
What Changes the SizeC5.1
The same reaction can give a big or small temperature change. The idea: the same heat shared through more water = smaller change; through less water = bigger change. Classic case: magnesium + excess acid. The magnesium runs out (it's limiting), so the same heat is released every time — pour in more acid and that fixed heat is just spread thinner, so the rise is smaller. Plot results with a single line of best fit, and to read a value just past the points, extend the line (extrapolate) and read across — with the unit.
> 🔑 Key idea: Same heat, more water → smaller change. Use a line of best fit, and extend it to read past the points. 📊
Worked Example: Off the ChartC5.1
Problem A best-fit line of temperature change against volume of acid goes up to 40 cm³. How do you find the value at 45 cm³?
Step 1 45 cm³ is past the points, so extend (extrapolate) the best-fit line smoothly to 45 cm³.
Step 2 Go up from 45 cm³ to the line, then across to the temperature axis, and read the value in °C.
Extend first, then read across, then write the unit. 📐
Key Points to Remember
- Exothermic = energy out, surroundings warm up. Endothermic = energy in, surroundings cool down. Temp up = exo, temp down = endo.
- Examples — exothermic: combustion, neutralisation, metal + acid, respiration. Endothermic: thermal decomposition, photosynthesis, dissolving ammonium nitrate.
- On a pathway diagram: products below reactants = exothermic; above = endothermic. The gap = energy transferred; bigger gap = more energy.
- Temperature change = final − initial. Subtract, never add; keep the decimal places.
- Energy (J) = volume of water (cm³) × 4.2 × temperature change (°C). Use the volume of water and the change, not the mass of solid or the final temperature.
- Fair test: keep everything the same except the one thing tested, and measure one outcome (time, or temperature rise).
- Heat loss makes the measured change too small; lagging, a lid, or a polystyrene cup help. It never flips exothermic to endothermic.
- Same heat, more water → smaller change; more reactant → bigger change. Use a line of best fit and extend it to read past the points.